Solar PV Water Heater Challenge

2018 February 22
by Chris Vernon

The Challenge
To ‘dump’ as much excess solar photovoltaic energy into hot water tank as possible whilst minimising impact on battery store.

Our house is off-grid, powered by 6.5 kWp solar photovoltaics and a wood stove with back boiler. Being off-grid, we can’t export electricity so it either has to be used as it’s being generated or stored in batteries. Once the batteries are full, it’s a case of use it or lose it. We want to store this otherwise lost energy as heat our hot water tank.

2x Victron BlueSolar MPPT 150/70
Colour Control GX
2x Phoenix Inverter 3000VA
360 Copper Thermal Store with 3x 2.25” immersion heater bosses
22kWh 48V Lead acid battery store

First question AC or DC. I think it’s better to use 48V DC immersion heater(s) so as not to use the inverters unnecessarily or even keep them on all the time (we have a smaller 1.2 kVA inverter which is on all the time). The thermal store is only about 10m from the battery store so cabling is not unreasonable.

The MPPT, BMV and CCGX all have relays with the following options available:

BlueSolar MPPT


Colour Control GX
Has a relay, but can only work on information provided by the other devices so maybe there’s no point in using it? However, it does have a neat way to start/stop a generator, track hours run etc. I wonder if this assistant could be used to start/stop the immersion heaters?

I don’t yet own the 48V immersion heaters, but say I get 3x 1.5kW for 4.5kW total. These could take a single cable then be hooked up in parallel at the water tank. This would be around 90A, over 10m so 14mm2 (6 AWG) for a 5% voltage drop. That’s some thick cable, and a single on/off or 4.5kW is not very delicate! Maybe better to run a separate cable to each immersion (more manageable 4.5mm (11 AWG) and have the choice of 1.5, 3 or 4.5 kW depending on how much excess solar was incoming.

Which relay(s) to use?
What to trigger on?
Relay minimum close time?
Is there a way of varying the power to the heaters, such that if there’s only 500W of ‘spare’ solar, that’s all that’ll get dumped?

My initial thought was to use the MPPT’s float option. Something like when the controller enters float, close the relay for 30 minutes. But this misses a lot of energy in the absorption stage before the batteries have reached float and there’s the risk of switching on the heaters late in the afternoon, when float has just been reached but there isn’t a large surplus. So maybe the BMV’s state or charge or battery voltage are better triggers? Battery voltage is at it’s highest during the absorption stage though so any high voltage trigger would switch on before the batteries are full?

I’ll also need high current DC-DC relays, something like this?
D06D100 – Solid State Relay, SPST-NO, 100 A, 60 VDC, Panel, Screw, DC Switch
Anything better/cheaper?

2 Responses leave one →
  1. Chris Vernon permalink*
    February 23, 2018

    Proposal, use voltage. Have three 1.5 kW (or 1kW) immersion heaters on separate relays and cables.

    First from the BMV using ‘default mode’ and high voltage thresholds 20 and 21 set such that the relay closes when the voltage reaches 56V (likely during bulk charge) and opening again if the voltage drops to 51.2V (equivalent to 12.8V on a 12V system indicating that the load is on the battery rather than being met from the PV). Set option 14 (minimum close time) to something like 5 minutes to reduce rapid switching.

    Second from the one of the MPPT charge controllers, option 7 and thresholds 14 and 15 such that the relay closes at 56.3V (slightly higher so it doesn’t come in at the same time as the first one) and off at the same 51.2V, again with a minimum close time of 5 minutes.

    Third from the other MPPT with either identical settings or a slightly higher relay close voltage if that’s possible given they the two are synchronised and the manual says settings have to be identical.

    Good points: When the batteries are well on the way to being full the immersion heaters will switch on one after another. If there isn’t enough ‘surplus’ solar energy coming in, this load will be partially met from the batteries pulling the voltage down below 51.2V and disconnecting the load (after a few minutes).

    Bad points: There has to be at least 1.5 kW (or 1kW) spare energy in order to put anything in the water. If there’s only a 500W surplus, that can’t be used. This configuration requires three separate solid state relays and they aren’t cheap.

  2. February 23, 2018

    Hi Chris.
    I think you could maybe use a contactor as used in machinery as the big current handling relay. Telemechanique make a whole range of them with different voltage triggering coils.

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